I'm just throwing it out there. I'm not going to lvl that way because I feel you'll miss too much in the game world by doing so. I just think they should make it to where our hits on them have no effect on your lvl'ing. They'd probably have to re-write a lot of iteration in the game in order to do.
I think that would be a terrific idea. Just make it so you don't gain skill from attacking followers. That way If npc becomes followers then if player attacks follower then exp = exp + 0. I've got some experience with programming and that how I imagine that would work out.